\(\dfrac{12}{16}=\dfrac{a}{20}=\dfrac{9}{b}=\dfrac{3}{4}=\dfrac{a}{4}=\dfrac{3}{b}\) (rút gọn)

\(=>\dfrac{a}{4}=\dfrac{3}{4}=\dfrac{15}{20},\dfrac{3}{b}=\dfrac{3}{4}=\dfrac{9}{12}\)

\(=>a=15,b=12\)

\(\text{1 +2 - 3 - 4 + 5 + 6 - 7 - 8 + 9 + ....... + 402 - 403 - 404 + 405+406}\)

\(=406+(405-404-403+402)+(401-400-399+398)+.........+(5-4-3+2)+1\)

\(=\text{406 + 0 + 0 + ....... + 0 + 1}\)

\(=407\)

rei tran thang nhe

\(-64.\left(-0,5\right)^7\)

\(=-64.\left(\dfrac{1}{2}\right)^7\)

\(=-64\cdot\dfrac{1}{128}\)

\(=-\dfrac{1}{2}\)

\(\dfrac{2}{11}:\dfrac{1}{3}x\dfrac{3}{2}=\dfrac{6}{11}x\dfrac{3}{2}=\dfrac{9}{11}\)

\(\dfrac{15}{24}+\dfrac{3}{4}-\dfrac{1}{6}=\dfrac{11}{8}-\dfrac{1}{6}=\dfrac{29}{24}\)

\(\dfrac{5}{2}x\dfrac{1}{3}+\dfrac{1}{5}=\dfrac{5}{6}+\dfrac{1}{5}=\dfrac{31}{30}\)

\(2\dfrac{3}{8}-1\dfrac{1}{4}+5\dfrac{1}{3}=\dfrac{19}{8}-\dfrac{5}{4}+\dfrac{16}{3}=\dfrac{9}{8}+\dfrac{16}{3}=\dfrac{155}{24}\)

\(\dfrac{7}{13}x\dfrac{5}{14}x\dfrac{39}{15}=\dfrac{5}{26}x\dfrac{39}{15}=\dfrac{1}{2}\)

\(\left[\dfrac{5}{2}+\dfrac{1}{4}\right]:\left[1-\dfrac{1}{3}\right]=\dfrac{11}{4}:\dfrac{2}{3}=\dfrac{33}{8}\)

\(\left[\dfrac{3}{4}-\dfrac{1}{5}\right]x\dfrac{9}{2}-\dfrac{1}{7}=\dfrac{11}{20}x\dfrac{9}{2}-\dfrac{1}{7}=\dfrac{99}{40}-\dfrac{1}{7}=\dfrac{653}{280}\)

\(\dfrac{18}{13}x\left[\dfrac{9}{4}-\dfrac{2}{5}\right]x\dfrac{1}{2}=\dfrac{18}{13}x\dfrac{37}{20}x\dfrac{1}{2}=\dfrac{330}{130}x\dfrac{1}{2}=\dfrac{330}{260}\)

so so hang cua day so tren la

\(\left(2015-1\right):1+1=2015\left(so\right)\)

tong day so tren la

\(2015x\left(2015+1\right):2=2031120\)

trung binh con cua da so tren la

\(2031120:2015=1008\)

\(x\) la dau nhan

Trung binh cong cua cac so tren la

\(\left(\text{2+6+10+14+18+22+26+30+34+38}\right):10=20\)

anh ơi câu cuối kết ra là 3 mà